Core drill

ABSTRACT

A double-tube core drill wherein the inner tube ( 2 ) is connected to the outer tube ( 1 ) by a multi-element needle and/or ball bearing ( 11 ), allows for the inner tube ( 2 ) to be made rigid and prevents it from rotating along with the outer tube ( 1 ). The extraction device ( 9 ) and the inner wall ( 38 ) of the tube are preferably covered with an antifriction coating. A set of stabilizing elements ( 67 ) are spread along the outer tube to stabilize the inner tube ( 2 ). Said inner tube has a tapered shoe ( 32 ) provided with a lower bevelled lip ( 14 ). The drill also comprises a device for indicating that the core has jammed in the inner tube ( 2 ) and a sealing device ( 19 ) designed to receive a ball ( 46 ) provided for sealing the inner tube ( 2 ) of said core drill.

BACKGROUND OF THE INVENTION

The present invention relates to a core drill, in particular for oilprospecting, comprising a core bit, an outer tube supporting the corebit so as to drive its rotation for core drilling, and an inner tubemounted in the outer tube so that it can receive a core sample cut bythe bit. A flow space provided between the outer and inner tubes isintended for the passage of a coring fluid to be conveyed to the bottomof a hole during core drilling.

There is a constant need to improve the operation of core drills, forexample by adding to them auxiliary devices which improve thereliability of the operations of taking hold of a core sample which hasbeen cut around its periphery and which has to be detached from thebottom of the bore hole in order to bring it up to the surface. The wayin which the auxiliary device or devices themselves work must, for itsown part, be controllable from the surface so that they can act at thedesired moment and in the intended way, with the lowest possible risk oflosing or damaging the core sample, damaging the core drill, etc, giventhe enormous cost involved in terms of labor and time for performingsuch core drilling operations which have therefore to be successfullycompleted as quickly as possible.

SUMMARY OF THE INVENTION

The object of the present invention is to provide simple and effectivemeans for controlling these auxiliary devices without introducingsophisticated and/or delicate mechanisms in the core drill but makingbest use of the possibilities, known as being such, offered by the useof modifications to the pressure of the core drilling fluid.

To this end, in the core drill of the invention, means for restrictingthe passage of core drilling fluid are arranged in the flow space at thefront end side of the inner tube, viewed according to a direction ofadvance of a core sample, and in addition, control means are providedfor adjusting, from the surface, the restricting means so as tosubstantially increase the pressure of said fluid upstream of therestricting means.

This increase in the pressure of the core drilling fluid can thus beexploited directly and effectively at the front end of the core drill inorder to actuate the aforementioned auxiliary devices there.

According to one embodiment of the invention, the passage-restrictingmeans comprise for this purpose an element of inner tube and an elementof outer tube which elements collaborate to produce the adjustablerestriction, and the control means are designed to move the inner andouter tubes one with respect to the other.

In another embodiment of the invention, the passage-restricting meanscomprise for this purpose an element of inner tube and/or an element ofouter tube and an auxiliary element designed to collaborate with theelement of inner tube and/or the element of outer tube with a view toadjusting the restriction. The control means may therefore be designedfor this purpose to move the inner and/or outer tubes and/or theauxiliary element one with respect to the other.

As a preference, in the core drill of the invention, at least one ofsaid elements of inner and/or outer tubes is an annular boss of whichone peripheral surface facing toward the other tube element collaborateswith a peripheral surface of this other element to form the restriction.

Advantageously, the control means may be designed to move the inner andouter tubes longitudinally one with respect to the other with a view toadjusting the restriction, the inner tube preferably being moved towardsthe front end of the core drill during this adjustment. For thispurpose, the control means may comprise, on the rear end side of theinner tube, a cylinder and piston assembly with a relatively limitedtravel, one of the piston and cylinder being secured to the inner tubeand the other being secured to the outer tube. A catch may be fitted soas to be able to lock the piston in the cylinder in a positioncorresponding to a chosen minimum restriction (relatively largepassage). In addition, the catch may be fitted so as to unlock thepiston from the cylinder when a determined pressure of core drillingfluid, higher than the core-drilling pressure, is applied to the catch.The limited relative travel of piston relative to cylinder as a resultof a pressure which is still higher, brings then the restricting meansfrom the front end of the inner tube into a position of chosen maximumrestriction (relatively small passage).

In the core drill of the invention, one type of auxiliary device maycomprise, arranged coaxially in the front end of the inner tube, asleeve with deformable wall which, in the undeformed condition, allows acore sample to pass, and a substantially impervious annular chambercontained between the inner tube and the wall of the sleeve, thischamber being in communication with the core drilling fluid passingthrough the flow space between the outer and inner tubes. The wall ofthe sleeve is chosen to deform toward the inside of the inner tube untilthe internal space thereof is substantially closed up and/or closed soas to retain a core sample located therein, under the pressure of saidfluid which pressure is obtained in the annular chamber after anaforementioned adjustment of the pressure by the chosen maximumrestriction.

Another type of auxiliary device for the core drill of the invention maycomprise, arranged coaxially in the front end of the inner tube, asliding ring allowing the passage of a core sample and mounted in themanner of a piston in a substantially impervious annular chambercontained between the inner tube and this ring, this chamber being incommunication with the core drilling fluid passing through the flowspace between the outer and inner tubes. The sliding ring is thendesigned to, on the one hand, occupy a first position slid towards thefront end of the core drill and be preferably locked therein by a catch,and, on the other hand, be slid into a second position away from thefront end of the core drill under the pressure of said fluid whichpressure is obtained in the annular chamber after the increase in theaforementioned pressure by the chosen maximum restriction, after anycatch there might be has released the ring under the action of saidpressure on the ring.

Further details and particulars of the invention will become apparentfrom the secondary claims and from the description of the drawings whichare appended to this description and which illustrate, diagrammaticallyand in longitudinal section, with cutaway and possibly different scales,and as non limiting examples, some advantageous embodiments of coredrills according to the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a front end of one embodiment of the core drill of theinvention, equipped with an auxiliary device.

FIG. 2 depicts one embodiment of a portion of the core drill of theinvention at the location of the rear end of the inner tube.

FIG. 3 depicts a front end of one embodiment of the core drill of theinvention, equipped with another auxiliary device.

FIG. 4 depicts a front end of one embodiment of the core drill of theinvention, equipped with a combination of two auxiliary devices.

FIG. 5 depicts a front end of one embodiment of the core drill of theinvention, equipped with another combination of two auxiliary devices.

FIG. 6 depicts another embodiment of the portion of the core drill ofthe invention at the location of the rear end of the inner tube.

FIG. 7 depicts a front end of another embodiment of the core drill ofthe invention.

DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

In the various figures, the same reference notation is used to denoteelements which are identical or similar.

The core drill 1 of the invention usually comprises (FIGS. 1, 3, 4, 5and 7), a core bit 2, an outer tube 3 supporting the bit 2, so as todrive its rotation for core drilling, and an inner tube 4 mounted in theouter tube 3 in such a way that it can take a core sample (not depicted)cut by the core bit 2. A flow space 5 provided, among other things,between the outer 3 and inner 4 tubes is intended for the passage of acore drilling fluid to be conveyed to the bottom of a hole during coredrilling, through nozzles pierced in the bit 2.

According to the invention, the core drill 1 additionally comprises, onthe one hand (FIGS. 1, 3, 4, 5 and 7), means 6 for restricting thepassage of fluid, which means are arranged in the flow space 5, on thesame side as or near to the front end 7 of the inner tube 4, viewedaccording to a direction of advance S of a core sampling and of the flowof core drilling fluid in this space 5 and, on the other hand (FIGS. 2and 6) control means 8 which are designed, preferably at the rear end 9side of the inner tube 4, for adjusting the restricting means 6 from thesurface so as to be able to substantially increase a pressure of saidfluid upstream of these restricting means 6.

According to one preferred embodiment of the invention, thepassage-restricting means 6 for this purpose comprise an element 10 ofthe inner tube 4 and an element 11 of the outer tube 3 which collaborateto produce the adjustable restriction. The control means 8 are thendesigned to move one with respect to the other the inner 4 and outer 3tubes to which the elements 10 and 11 respectively are attached.

Given that the inner 4 and outer 3 tubes usually turn one with respectto the other about their known longitudinal axis, it is advantageous forthe control means 8 to be arranged in such a way as to move the inner 4and outer 3 tubes longitudinally one with respect to the other with aview to adjusting the restriction, the inner tube 4 preferably beingmoved towards the front end 7 of the core drill 1 (in the direction ofthe arrow S) during this adjustment.

According to another embodiment of the invention, thepassage-restricting means 6 may for this purpose comprise anaforementioned inner tube element 10 and/or an aforementioned outer tubeelement 11 and an auxiliary element, not depicted, designed tocollaborate with the inner tube element 10 and/or the outer tube element11 for adjusting the restriction. This auxiliary element could be a ringplaced between the inner 10 and outer 11 tubes, in the flow space 5. Inthis case, the control means 8 may then be designed to move the inner 4and/or outer 3 tubes and/or the auxiliary element one with respect tothe other.

For the reasons mentioned hereinabove, in the case of this otherembodiment, the control means 8 may be designed to move the auxiliaryelement longitudinally with respect to the inner 4 and/or outer 3 tubeswith a view to adjusting the restriction.

To make the restricting means 6 easy to shape, at least one of the twoelements 10, 11 of inner 4 and/or respectively outer 3 tubes is anexternal annular boss 13 or respectively internal annular boss 14. Forexample, an external peripheral surface of the external annular boss 13or of the element 10 itself, facing towards the other tube element 11,collaborates with an internal peripheral surface of this other element11 or of its annular boss 14 to form the restriction.

Likewise, the aforementioned auxiliary element could be equipped with aninternal annular boss and/or with an external annular boss designed tocollaborate, respectively, in order to obtain the restriction, with theinner tube element 10 which may or may not be equipped with acorresponding boss, and/or with the outer tube element 11 which likewisemay or may not be equipped with a corresponding annular boss.

To actuate the restricting means 6 explained hereinabove, by means of anaforementioned longitudinal movement, the control means 8 may comprise,on the rear end 9 side of the inner tube 4, a system which will be knownas a cylinder 22 and piston 23 system with a relatively limited travelbecause, among others, one is arranged to slide inside the other. Forexample, the cylinder 22 (FIG. 2) is secured to the inner tube 4 and thepiston 23 is secured to the outer tube 3 via the thrust ball bearing 25designed for suspending the inner tube 4 inside the outer tube 3. Acatch 24 is advantageously provided to prevent any relative longitudinalmovement of the piston 23 with respect to the cylinder 22 when theseelements are in a starting position corresponding to usual core drillingwith a chosen minimum restriction. The catch 24 is then designed to beable to unlock the cylinder 22 from the piston 23 when a determinedpressure of the core drilling fluid, higher than that of normal coredrilling, is applied to the catch 24.

The catch 24 of FIG. 2 may, for example, comprise a tube 26 which canslide in the piston 23, and balls 27 housed in radial cylindrical holes,through the wall of the piston 23 and projecting into appropriateindentations, or into an annular groove 28, cut in the internal face ofthe cylinder 22. An external annular boss 29 of the tube 26 is arrangedthereon at a location in which, with the entire device in theconventional core drilling position, it keeps the balls 27 locked in theposition described hereinabove so as to form a connection between thecylinder 22 and the piston 23. A locking rod 30, fixed through thepiston 23 and the sliding tube 26, holds the latter in the position inwhich it locks the balls 27. At its rear end (viewed according to thedirection S), the sliding tube 26 has a valve seat 31 intended, as aclosure valve, to accommodate a ball 32 and is inserted leaktightly inthe core drilling fluid inlet conduit 33 in this position for lockingthe balls 27. A first set of fluid passages 34 and a second set of fluidpassages 35, all transversal to the wall of the sliding tube 26, areeach located at a different level therein.

During normal core drilling, the core drilling fluid from the conduit 33passes through the valve seat 31, flows into the sliding tube 26 andemerges therefrom via, among other things, the set of passages 34 toemerge, via flow holes 36, in the aforementioned flow space 5.

When it is desired for maximum restriction to be commanded at the frontend 7 from the surface, the ball 32 is thrown into the path of the fluidand becomes seated on the valve seat 31 and thus blocks off the normalflow of core drilling fluid. The pressure of the fluid therefore exertsits full effects on the ball 32 and on the entire cross section of thesliding tube 26, visible at its rear end inserted in the conduit 33.When said pressure increases through lack of escape flow across thevalve seat 31, it is able to produce sufficient force on the slidingtube 26 to shear the locking rod 30. The sliding tube 26 thus releasedand driven back by the pressure slides in the piston 23 until it comesup against a stop as far as a point at which its annular boss 29releases the locking balls 27 which therefore detach the piston 23 fromthe cylinder 22.

At the end of the sliding travel under the pressure of the fluid, thesliding tube 26 leaves the conduit 33. The core drilling fluid cantherefore escape by passing around the rear end thus released of thesliding tube 26 and it enters the latter through the set of passages 35or alternatively passes around the sliding tube 26 in an annular gapbetween this tube and the piston 23, to once more reach the flow space 5via the flow holes 36.

The relative travel of the piston 23 in the cylinder 22 is limited, forexample, by a peg 37 fixed to the piston 23 and by an oblong hole 38 cutin the wall of the cylinder 22. The peg 37 allows the piston 23 to havea travel which is limited to the movement of the peg 37 from one end ofthis oblong hole 38 (as depicted in FIG. 2) to the other end of thissame hole 38.

This limited relative travel is brought about by the pressure of thefluid on the outside of the inner tube 4. The latter, moved in thedirection S, brings the restricting means 6 into a chosen maximumrestriction position.

The core drill 1 according to the invention may comprise (FIG. 1) a kindof auxiliary device 40 intended, for example, to grip onto a core samplethat is to be grasped in order to bring it to the surface. Thisauxiliary device 40 arranged coaxially in the front end 7 of the innertube 4 may comprise a sleeve 41 with deformable wall 42, which allowsthe core sample to pass when it is in an undeformed starting condition.A substantially impervious annular chamber 43 of the device 40 isarranged between the inner tube 4 and the deformable wall 42 of thesleeve 41, this chamber 43 being in communication, via one or more holes44 in the wall of the inner tube 4, with the core drilling fluid passingthrough the flow space 5. The wall 42 is chosen to deform toward theinside of the inner tube 4, until such time as it substantially closesup and/or closes the internal space 45 thereof so as to hold in a coresample located there, under the pressure of said fluid which pressure isobtained in the annular chamber 43 after an aforementioned adjustment ofthe pressure using the chosen maximum restriction.

The core drill 1 according to the invention may comprise (FIG. 3)another kind of auxiliary device 49 intended, for example, to form arelatively plain passage at the front end 7 of the inner tube 4,particularly at the location of known means 50 used for gripping and/orgrasping a core sample, such as a split frustoconical ring. Theauxiliary device 49 may comprise, arranged coaxially in the front end 7of the inner tube 4, a sliding ring 51 which allows a core sample topass and is mounted in the manner of a piston in a substantiallyimpervious annular chamber 52 contained between the inner tube 4 andthis ring 51. The chamber 52 communicates, via one or more holes 53pierced in the inner tube 4, with the core drilling fluid passingthrough the flow space 5 between the outer 3 and inner 4 tubes. Thesliding ring 51 can occupy a first position (shown in FIG. 3) slidtoward the front end 54 of the core drill 1, and preferably can belocked in this position by a catch 55, and can be slid into a secondposition away from the front end 54 of the core drill 1 under thepressure of said fluid which pressure being obtained in the annularchamber 52 in communication with the hole(s) 53 after the pressure hasbeen increased by the chosen maximum restriction. This takes place afterany catch 55 that may be present has released the ring 51 under theaction of said pressure on the ring 51. This catch 55 may be a lockingrod which breaks under the force of a chosen pressure, acting to causethe ring 51 to slide.

The ring 51 may comprise, when viewed along its length and in successionfrom its rear end to its front end,

a relatively thin wall 56 concealing an open section of the annularchamber 52, in said first position,

a circular flange 57 acting as a piston head,

a relatively thick wall 58 to withstand the pressure of the fluid andwhich collaborates with the inner tube 4 to form the annular chamber 52,and

a wall 59, preferably a relatively thin wall, intended to conceal fromthe core sample in the inner tube 4 the means 50 which are designed togrip this sample in order to extract it from the bore hole.

The inner tube 4 may then comprise an internal shoulder 60, opposite theflange 57 of the ring 51 and intended to close the annular chamber 52.The longitudinal dimension of the annular chamber 52 is chosen so thatthe travel of the ring 51 therein allows the means 50 intended to gripthe core sample to be released so that these means can act.

Advantageously, the inner tube 4 is fixed to the outer tube 3 in such away that if the core sample should become jammed in the inner tube 4,the latter can be pushed back toward the rear end of the core drill 1.In this case, it is practical for the inner 4 and outer 3 tubes tocomprise throttling means 65 which collaborate when the inner tube 4 ispushed back, so as to increase the pressure of the core drilling fluid.Such an increase in pressure can be interpreted at the surface as beinga signal that there is a jam in the inner tube 4.

Said throttling means 65 may be combined (FIGS. 3 and 4) with theaforementioned element 10 of inner tube 4 and/or that 11 of the outertube 3. The latter therefore preferably has, on the internal annularboss 11, a circular face 66 which collaborates with an additionalexternal annular boss 67 of the inner tube 4 to perform theaforementioned throttling when the inner tube 4 is driven back insidethe outer tube 3 by a core sample, this tube being suspended in theouter tube 3 only by the thrust ball bearing 25 pushed back against theouter tube by the pressure of the fluid.

In FIG. 2, the throttling means 65 are shown arranged on the rear endside of the inner tube 4, near the point at which it is suspended in theouter tube 3. The flow holes 36 open into the flow space 5 near the wallof the outer tube 3. Higher up (according to the drawing) than the flowholes 36, the wall of the outer tube 3 has an inside diameter which issmaller than it is below or even with these flow holes 36 when they arein a conventional core-drilling relative position of the inner 4 andouter 3 tubes. As the inner tube 4 is suspended by the thrust ballbearing 25 resting against the outer tube 3, if the inner tube 4 ispushed back in the opposite direction to the direction S, in the outertube 3 as a result of a core sample becoming stuck in the inner tube 4or for any other reason, the flow holes 36 come opposite the smallestdiameter of the outer tube 3. The fluid is throttled and its pressurerises and this increase in pressure can again be interpreted at thesurface.

It goes without saying to the person skilled in the art that in theabove explanations it may be considered that the outer tube 3 and thecoring bit 2 can form just one assembly, at least as far as thefunctions and internal elements thereof are concerned. Thus, elementsdescribed hereinabove as forming part of the outer tube 3 may, however,be in the coring bit 2, whether this be in the drawings or in theembodiments not depicted in these drawings.

In addition, the inner tube 4 may be considered as comprising the partswhich are suspended in the outer tube 3 via the thrust ball bearing 25,the conduits 33 for conveying fluid, etc.

The aforementioned catch or catches 24, 55 may comprise or consist ofone or more bodies of material and cross section chosen to break underthe action of the pressure corresponding to the unlocking considered.

It is to be understood that the invention is not in any way restrictedto the embodiments described and that many modifications can be made tothe latter without departing from the scope of the claims appended tothis description.

Thus, in the embodiment of FIG. 6, the catch 24 comprises a sliding tube26 which differs somewhat from the previous one.

When the ball 32 arrives on the valve seat 31 of this sliding tube 26and the pressure of the fluid applied to it breaks the locking rod 30,aside from operating in the way described hereinabove, the sliding tube26 continues its longitudinal travel and rests on the cylinder 22, toassist the latter in causing the inner tube 4 to advance in thedirection S.

Before the ball 32 was sent into the conduit 33, the core drilling fluidwas flowing from the conduit 33 across the valve seat 31 and through theflow holes 36 as far as the flow space 5. After the ball 32 has closedthe valve seat 31, and the sliding tube 26 has completed its travel, thefluid passes from the conduit 33 toward passages 70 and then, throughthe gap between the outer 3 and inner 4 tubes, to the flow space S.

FIG. 4 shows a combination of the means 50 for gripping the core sampleand of the auxiliary devices 49 and 40 explained hereinabove. Startingfrom the front end 54 of the core drill 1, we find the gripping means50, and an auxiliary device 49 designed to conceal the latter from acore sample so that this core sample “sees” a relatively plain passageand, above that, another auxiliary device 40 with a gripping sleeve 41as described above.

FIG. 5 shows a combination, which is the reverse by comparison with thatof FIG. 4, of the same means 50 for gripping the core sample andauxiliary devices 49 and 40.

FIGS. 1, 3, 4, and 5 show the invention in the case of an inner tube end75 which is thinned in terms of its thickness and arranged in an annulargroove 76 extending in the direction of the axis of the core drill 1toward the bottom of the bore hole. FIG. 7 shows the invention in adifferent configuration of the end 75 of the inner tube with respect tothe bore 77 of the coring bit 2, it being possible for the latter end 75to then be brought closer to the bottom of the bore hole than the endpushed into the groove 76.

KEY TO THE REFERENCE NUMERALS USED IN THE FIGURES

1 core drill

2 core bit

3 outer tube

4 inner tube

5 flow space

6 means for restricting the passage of fluid

7 front end of 4

8 control means

9 rear end of 4

10 element of the inner tube 4

11 element of the outer tube 3

13 external annular boss of 10

14 internal annular boss of 11

22 cylinder

23 piston

24 catch

25 thrust ball bearing

26 sliding tube

27 locking balls

28 scallops or annular groove

29 external annular boss of 26

30 locking rod

31 valve seat

32 valve ball

33 inlet conduit

34 fluid passage(s) of 26

35 fluid passage(s) of 26

36 flow hole(s)

37 peg of 23

38 oblong hole of 22

40 auxiliary device

41 grasping sleeve

42 deformable wall

43 annular chamber

44 hole(s) in 4

45 internal space of 4

49 another kind of auxiliary device

50 means for gripping and/or grasping a core sample

51 sliding ring

52 annular chamber

53 hole(s) in 4

54 front end of 1

55 catch

56 wall of 51

57 circular flange of 51

58 wall of 51

59 wall of 51

60 internal shoulder of 4

65 throttling means

66 circular face of 11

67 additional external annular boss of 4

70 passage(s)

75 front end of the inner tube 4

76 annular groove of 2

77 bore of 2

S direction of advance

What is claimed is:
 1. A core drill, in particular for oil prospecting, comprising: a core bit (2), an outer tube (3) supporting the core bit (2) so as to drive the core bit rotation for core drilling, an inner tube (4) mounted in the outer tube (3) so that the inner tube can receive a core sample cut by the core bit (2), a flow space (5) provided between the outer (3) and inner (4) tubes and intended for the passage of a core drilling fluid to be conveyed to a bottom of a hole during core drilling, wherein passage restricting means (6) for restricting the passage of fluid are arranged in the flow space (5) at the front end (7) side of the inner tube (4), viewed according to a direction of advance (S) of a core sample, control means (8) are provided for adjusting, from the surface, the restricting means (6) so as to increase a pressure of said fluid upstream of the restricting means (6), the passage-restricting means (6) comprise an element (10) of inner tube (4) and an element (11) of outer tube (3) which elements (10 and (11) collaborate to produce an adjustable restriction, the control means (8) are designed to move the inner (4) and outer (3) tubes one with respect to the other, the control means (8) are designed to move the inner (4) and outer (3) tubes longitudinally one with respect to the other with a view to adjusting the adjustable restriction, and wherein, for the longitudinal movement, the control means (8) comprise, on the rear end (9) side of the inner tube (4), a cylinder (22) and piston (23) assembly with a limited travel, one of which cylinder (22) and piston (23) is secured to the inner tube (4) and the other of which cylinder (22) and piston (23) is secured to the outer tube (3), a catch (24) locks the piston (23) in the cylinder (22) in a position that corresponds to a chosen minimum restriction of said passage restricting means (6), and the catch (24) is designed to unlock the piston (23) from the cylinder (22) when a determined pressure of core drilling fluid, higher than the core-drilling pressure of said core drilling fluid is applied to the catch (24), the limited travel of piston (23) relative to cylinder (22) as a result of a pressure of core drilling fluid which is still higher than said determined pressure, bringing the restricting means (6) from the front end (7) of the inner tube (4) into a position of chosen maximum restriction of said passage restricting means (6).
 2. A core drill, in particular for oil prospecting, comprising: a core bit (2), an outer tube (3) supporting the core bit (2) so as to drive the core bit rotation for core drilling, an inner tube (4) mounted in the outer tube (3) so that the inner tube can receive a core sample cut by the core bit (2), a flow space (5) provided between the outer (3) and inner (4) tubes and intended for the passage of a core drilling fluid to be conveyed to a bottom of a hole during core drilling, wherein passage restriction means (6) for restricting the passage of fluid are arranged in the flow space (5) at the front end (7) side of the inner tube (4), viewed according to a direction of advance (S) of a core sample, control means (8) are provided for adjusting, from the surface, the restricting means (6) so as to increase a pressure of said fluid upstream of the restricting means (6), and which comprises, arranged coaxially in the front end (7) of the inner tube (4), a sliding ring (51) allowing the passage of a core sample and mounted in the manner of a piston (23) in an annular chamber (52) contained between the inner tube (4) and this ring (51), this chamber (52) being in communication with the core drilling fluid passing through the flow space (5) between the outer (3) and inner (4) tubes, the sliding ring (51) being capable of occupying a first position slid towards the front end (54) of the core drill (1), and being slid into a second position away from the front end (54) of the core drill (1) under the pressure of said core drilling fluid obtained in the annular chamber (52) after the increase in the aforementioned pressure of said core drilling fluid by the chosen maximum restriction, after the catch (55) there might be has released the ring (51) under the action of pressure of said core drilling fluid on this ring.
 3. A core drill, in particular for oil prospecting, comprising: a core bit (2), an outer tube (3) supporting the core bit (2) so as to drive the core bit rotation for core drilling, an inner tube (4) mounted in the outer tube (3) so that the inner tube can receive a core sample cut by the core bit (2), a flow space (5) provided between the outer (3) and inner (4) tubes and intended for the passage of a core drilling fluid to be conveyed to a bottom of a hole during core drilling, wherein passage restriction means (6) for restricting the passage of fluid are arranged in the flow space (5) at the front end (7) side of the inner tube (4), viewed according to a direction of advance (S) of a core sample, control means (8) are provided for adjusting, from the surface, the restricting means (6) so as to increase a pressure of said fluid upstream of the restricting means (6), and which comprises, arranged coaxially in the front end (7) of the inner tube (4), a sliding ring (51) allowing the passage of a core sample and mounted in the manner of a piston (23) in an annular chamber (52) contained between the inner tube (4) and this ring (51), this chamber (52) being in communication with the core drilling fluid passing through the flow space (5) between the outer (3) and inner (4) tubes, the sliding ring (51) being capable of occupying a first position slid towards the front end (54) of the core drill (1), and being slid into a second position away from the front end (54) of the core drill (1) under the pressure of said core drilling fluid obtained in the annular chamber (52) after the increase in the aforementioned pressure of said core drilling fluid by the chosen maximum restriction, after any catch (55) there might be has released the ring (51) under the action of pressure of said core drilling fluid on this ring and wherein, the ring comprises at least, when viewed in the direction of its length and, in succession, a circular flange (57) acting as a piston head, a wall (58) to withstand the pressure of the fluid and collaborating with the inner tube (4) to form the annular chamber (52), a wall (59) intended to conceal from the core sample in the inner tube (4) means (50) which are intended to grip this core sample so as to remove this core sample from the bore hole, the inner tube (4) comprises an internal shoulder (60) opposite the flange (57) of the ring, and intended to close the annular chamber (52), and the longitudinal dimension of the annular chamber (52) is chosen so that the travel of the ring (51) therein allows the inner tube means (50) intended to grip the core sample to be released.
 4. A core drill, in particular for oil prospecting, comprising: a core bit (2), an outer tube (3) supporting the core bit (2) so as to drive the core bit rotation for core drilling, an inner tube (4) mounted in the outer tube (3) so that the inner tube can receive a core sample cut by the core bit (2), a flow space (5) provided between the outer (3) and inner (4) tubes and intended for the passage of a core drilling fluid to be conveyed to a bottom of a hole during core drilling, wherein passage restricting means (6) for restricting the passage of fluid are arranged in the flow space (5) at the front end (7) side of the inner tube (4), viewed according to a direction of advance (S) of a core sample, control means (8) are provided for adjusting, from the surface, the restricting means (6) so as to increase a pressure of said fluid upstream of the restricting means (6), the passage-restricting means (6) comprise an element (10) of inner tube (4) and an element (11) of outer tube (3) which elements (10 and (11) collaborate to produce an adjustable restriction, the control means (8) are designed to move the inner (4) and outer (3) tubes one with respect to the other, the control means (8) are designed to move the inner (4) and outer (3) tubes longitudinally one with respect to the other with a view to adjusting the adjustable restriction, wherein, for the longitudinal movement, the control means (8) comprise, on the rear end (9) side of the inner tube (4), a cylinder (22) and piston (23) assembly with a limited travel, one of which cylinder (22) and piston (23) is secured to the inner tube (4) and the other of which cylinder (22) and piston (23) is secured to the outer tube (3), a catch (24) locks the piston (23) in the cylinder (22) in a position that corresponds to a chosen minimum restriction of said passage restoring means (6), the catch (24) is designed to unlock the piston (23) from the cylinder (22) when a determined pressure of core drilling fluid, higher than the core-drilling pressure of said core drilling fluid is applied to the catch (24), the limited travel of piston (23) relative to cylinder (22) as a result of a pressure of core drilling fluid which is still higher than said determined pressure, bringing the restricting means (6) from the front end (7) of the inner tube (4) into a position of chosen maximum restriction of said passage restricting means (6), and the aforementioned catch (24) comprises one or more bodies of material and cross section which are chosen to break under the action of the pressure corresponding to the unlocking in question. 